1. The Field of the Invention
The present invention relates to tunable solid state laser materials. More particularly, the present invention is directed to novel color centers in alkali halide crystals which are useful in tunable laser operation in the near infrared region of the optical spectrum.
2. The Prior Art
Various solid state laser materials have been developed in recent years that may be pumped in the visible or near ultraviolet regions of the optical spectrum by readily available ion lasers or the like, and which emit laser light at a longer wavelength. Inexpensive and easily operated laser sources at wavelengths in the near infrared region of the optical spectrum have been earnestly sought after because of their applications in atomic and molecular spectroscopy, photochemistry, and fiber optic communications.
The most promising approach to achieving such wavelength laser sources has been the development of color center laser materials. which incorporate point defects into alkali halide host crystals in order to introduce electronic transitions into the forbidden energy gap of the host crystal. In addition to emitting light in the near infrared region of the optical spectrum, color center materials are capable of producing laser light of extremely narrow linewidths, and yet, the lasers are wavelength tunable within relatively broad limits. As a result, investigators have considered color center lasers to be more promising than other tunable coherent light sources in the infrared region of the spectrum, such as diode lasers and optical parametric oscillators. Diode lasers are disadvantageous because they are capable of only relatively low output power, and also have high spatial beam divergence. Optical parametric oscillators avoid these problems, but are complex and quite expensive. Moreover, neither alternative system is capable of producing the narrow linewidths which may be obtained from color center lasers without significant losses of laser power output.
Color center lasers having various degrees of usefulness and efficiency have been developed utilizing alkali halide host crystals doped with lithium or sodium, which have been introduced into the host crystal so as to allow the formation and the stabilization of particular point defects.
In these alkali halide host crystals, most of the electronic transitions, which are induced by the point defects, are strongly coupled to the crystal lattice and its vibrations. As a result, the optical transition from the ground state to the excited state provides a new spatial distribution of the electron which is no longer in equilibrium with the lattice, thus exerting forces on the surrounding ions. This leads to strong lattice vibrations which are relieved by relaxation into a new electron-lattice configuration, termed the relaxed excited state. After the radiative lifetime, optical emission occurs, and a new non-equilibrium state occurs. Again, lattice vibrations result in relaxation, this time back to the relaxed ground state.
This strong coupling between the electron and the lattice, termed "electron-phonon coupling" results in relatively broad absorption and emission bands separated by a significant shift in wavelength (termed the "Stokes shift"). This Stokes shift is very advantageous because excitation may be accomplished at a wavelength emitted by readily available pump lasers, yet emission will occur at some longer wavelength.
Laser activity only occurs when an appropriate combination of impurity cation and host crystal is used. Heretofore, three basically different types of color centers have been developed which have been shown capable of laser activity. All three types depend upon the presence of "F centers," which are anionic vacancies in the crystal lattice filled by an electron.
One of the earlier types of color center, termed an "F.sub.A center," consists of an F center associated with an impurity cation. A second type, termed an "F.sub.B center," consists of an F center associated with two impurity cations. The third type of color center reported heretofore, termed the "F.sub.2.sup.+ center," consists of a pair of neighboring anion vacancies which share a single electron.
Laser materials of the F.sub.A or F.sub.B type have shown a relatively large Stokes shift, i.e., there is a rather large difference between the frequency of absorption and the frequency of emission. Although such large Stokes shifts generally result in low efficiency, workable and reliable lasers of these types have been demonstrated and are capable of tunable emission in the near infrared spectral range of about 2.2 to 3.3 micrometers.
The F.sub.2.sup.+ -type centers do not have such a large Stokes shift. As a result, they are more efficient and also allow selection of tunable emission at shorter wavelengths within the near infrared region. Unfortunately, these types of centers are optically and/or thermally unstable, and thus difficult and expensive to produce and handle. Although over a dozen different systems of this type have been found capable of laser operation, none have proven stable enough for routine use.
From the foregoing, it may be seen that much of the near infrared region of the optical spectrum remains uncovered by the emission bands of presently available stable color center lasers. Accordingly, it would be a significant contribution to the field of tunable lasers to provide new optically and thermally stable laser-active color centers having a Stokes shift into the near infrared region of the optical spectrum.